Photovoltaic devices are generally understood as photovoltaic cells or photovoltaic modules. Photovoltaic modules ordinarily comprise arrays of interconnected photovoltaic cells.
A thin-film photovoltaic or optoelectronic device is ordinarily manufactured by depositing material layers onto a substrate. A thin-film photovoltaic device ordinarily comprises a substrate coated by a layer stack comprising a conductive layer stack, at least one absorber layer, optionally at least one buffer layer, and at least one transparent conductive layer stack.
The present invention is concerned with photovoltaic devices comprising an absorber layer generally based on an ABC chalcogenide material, such as an ABC.sub.2 chalcopyrite material, wherein A represents elements in group 11 of the periodic table of chemical elements as defined by the International Union of Pure and Applied Chemistry including Cu or Ag, B represents elements in group 13 of the periodic table including In, Ga, or Al, and C represents elements in group 16 of the periodic table including S, Se, or Te. An example of an ABC.sub.2 material is the Cu(In,Ga)Se.sub.2 semiconductor also known as CIGS. The invention also concerns variations to the ordinary ternary ABC compositions, such as copper-indium-selenide or copper-gallium-selenide, in the form of quaternary, pentanary, or multinary materials such as compounds of copper-(indium, gallium)-(selenium, sulfur), copper-(indium, aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur), copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur), (silver, copper)-(indium, gallium)-selenium, or (silver, copper)-(indium, gallium)-(selenium, sulfur).
The photovoltaic absorber layer of thin-film ABC or ABC.sub.2 photovoltaic devices can be manufactured using a variety of methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), spraying, sintering, sputtering, printing, ion beam, or electroplating. The most common method is based on vapor deposition or co-evaporation within a vacuum chamber ordinarily using multiple evaporation sources. Historically derived from alkali material diffusion using soda lime glass substrates, the effect of adding alkali metals to enhance the efficiency of thin-film ABC.sub.2 photovoltaic devices has been described in much prior art (Rudmann, D. (2004) Effects of sodium on growth and properties of Cu(In,Ga)Se.sub.2 thin films and solar cells, Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved 2014 Apr. 30 from <URL: http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.-pdf>).
The present invention presents a method to form nanostructures, such as cavities, at the surface of a photovoltaic device's absorber layer by selectively dissolving alkali crystals embedded within the absorber's surface. The method advantageously uses the cavities to modify the absorber layer surface's chemical composition, enlarge developed total surface, enlarge developed surface adequate for receiving doping elements, and form point contacts with subsequently deposited thin-film layers.
The present invention exploits adding at least one alkali metal to a thin-film optoelectronic device, and especially to its absorber layer. A preferred at least one alkali metal comprises potassium. Adding at least one alkali metal modifies at least the absorber layer's chemical content. It may also modify the physical appearance of the surface of the absorber layer. Further treating of at least the surface of the absorber layer will modify its physical appearance to reveal nanostructures. Treating of absorber surface may for example be done with a bathing apparatus. The invention discloses independent control of separate alkali metals during adding to layers of the optoelectronic device, the treating of the absorber surface, and the resulting chemical and physical modifications to at least one absorber layer of the optoelectronic device. Effects of the invention on at least one of the device's thin-film layers include at least one of doping, passivation of absorber surface, interfaces, grain boundaries, and defects, elemental interdiffusion, forming of point contacts, forming of nanoholes, modification of layer roughness, optical characteristics, and optoelectronic characteristics such as enhanced open circuit voltage and fill factor. The invention's adding of at least one alkali metal and treating absorber surface enables manufacturing of a thinner optimal buffer layer. In some cases a person skilled in the art may advantageously omit manufacturing the buffer layer. This thinner optimal buffer layer results in reduced optical losses, thereby contributing to increase the device's photovoltaic conversion efficiency.